There is increasing interest in substituting high performance lightweight reinforced composite components and structures comprising a polymer matrix with a suitable reinforcement for stamped sheet metal components in vehicles. Suitable polymers are often thermosets, such as epoxies, vinyl-esters or polyesters, or thermoplastics, such as polypropylene or poly amide, and suitable reinforcements include structural fibers such as carbon, glass or aramid fibers. Such fibers may be randomly oriented and arranged or aligned along one or more preferred directions.
Individual carbon fibers may range from about 5 to 10 micrometers in diameter with 7 micrometer diameter fibers being especially common. Individual glass fibers may range from about 7 to 30 micrometers, depending in part on the grade of glass. In many applications, particularly those employing aligned fibers, assemblages of commonly-oriented fibers, variously called tows or roving, are used. Such carbon fiber assemblages may contain as few as 1000 or as many as 50,000 or more fibers, while glass fiber assemblages may include up to 200,000 or more fibers.
For fabric applications, aligned fibers may be assembled into one of two fabric structures for ease of application: a woven cloth or a non-woven fabric often called a non-crimp or stitch bonded fabric. A woven cloth employs tows of a first orientation which alternately overlie and underlie fiber tows of a second orientation, usually at about 90° to the first orientation. The weave may be tight, with adjacent tows positioned about a millimeter or less apart, or loose, with adjacent tows spaced up to about 10 millimeters apart. In an alternative structure, a number of spaced apart fiber rovings, individually fed from their respective spools, may be simply laid alongside one another in a ply, and temporarily secured and locked into place, by stitching, using, for example, a polyester yarn. Such stitching generally extends over the length and breadth of the reinforcement ply and is usually accomplished with a stitch beam which incorporates a plurality of needles and has a suitable motion to enable both simple chain stitches and other more complex stitches, for example tricot stitches. In many cases multiple coextensive plies are laid atop one another and the rovings of all of the plies are secured in a single stitching operation. Often the plies are placed with the fiber orientations of adjacent plies rotated one from another to render the in-plane properties less directional, or more isotropic, in the multi-ply reinforcement than in each ply individually. The weight of each ply is determined by the bulk of the roving and the spacing between adjacent roving bundles. These, non-woven reinforcements are called stitch bonded fabrics or non crimp fabrics, often abbreviated as NCF.
One common example of a multi-ply NCF is a 4-layer grouping of fibers arranged at 0°, +45°, −45° and 90° respectively with substantially equal numbers of fibers in each orientation. A 2-layer NCF with fibers arranged at +45° and −45° also finds wide application. Of course this description of such a multi-ply NCF is intended to be exemplary and not limiting. It will be appreciated that variations in the number of plies, in the number of orientations, in the angular alignment of the fibers within any ply and in the fiber density in each orientation are comprehended by the terminology non-crimp fabric, stitch bonded fabric, NCF, NCF fabric or aligned fiber layer as used in this specification.
Such fabric reinforcements, woven or non-woven, may be impregnated with a suitable polymer resin, placed in a mold, shaped and then cured, typically at modestly elevated temperature, say about 150° C., to form the desired polymer composite. It will be appreciated that the above-listed sequence of operations may be modified for different molding processes. For example, preforms may be placed in a mold with resin already impregnated, or the resin can be added after the preform is in the mold via resin infusion, resin transfer molding, or structural resin injection molding. Thermoplastic or thermoset sheets or materials with comingled strands of thermoplastic and reinforcing fiber may also be employed.
Commonly, more than one fabric reinforcement may be required to develop the desired properties in the composite. These reinforcements may be stacked atop one another, while possibly rotating or offsetting one layer with respect to another, with the goal of developing greater isotropy, or lack of directionality in properties, at least in the plane of the reinforcement.
Reinforcing layers in which the reinforcing fibers are randomly oriented such as by directed fiber preforming or Programmable Powered Preform Process (P4™ preforming), or one or more layers of continuous strand mat such as Owens Corning 8610 or chopped strand mat also find application. Such reinforcements may, by virtue of the fibers being oriented over all possible orientations, offer more isotropic properties than even a multilayer NCF fabric reinforcement.
One suitable configuration for a multilayer fiber-based polymer composite reinforcement is a layer of randomly-oriented fibers sandwiched between two layers of aligned fibers, which may be assembled as NCF (non crimp fabric) layers or woven layers. But, such multilayer reinforcements are also multi-piece, and require that each reinforcement layer be placed and positioned individually, complicating manufacturing.
There is therefore need for a one-piece reinforcement which facilitates manufacturing of fiber reinforced polymer composite articles and at least meets the performance objective of multilayer, multi-piece reinforcements.